Acrylic acid is an important intermediate used in particular in the form of its polymers. The best-known use of acrylic acid is the production of superabsorbent polymers based on polyacrylic acid.
Acrylic acid is conventionally chemically produced by oxidation of propene to give acrolein and subsequent further oxidation to afford acrylic acid.
Newer methods seek to produce acrylic acid using other feedstocks. For instance, WO 2008/061819 describes a method for producing (meth)acrylic acid (the term (meth)acrylic acid is intended to encompass both methacrylic acid and acrylic acid) where cyclic esters are converted into (meth)acrylic acid in the presence of a catalyst.
In recent years, economic and ecological considerations have spurred numerous attempts to use other feedstocks to produce acrylic acid or methacrylic acid, in particular renewable feedstocks.
For instance, WO 2008/145737 describes the production of methacrylic acid by dehydration of 3-hydroxyisobutyric acid which was produced in biotechnological fashion from renewable feedstocks, in particular from carbohydrates and/or glycerol. The dehydration is performed in the liquid phase at a pressure in the range of from 200 to 500 mbar and a temperature in the range of from 200° C. to 230° C. in the presence of alkali metal ions as catalyst.
DE-A 1 768 253 likewise describes such a method for producing methacrylic acid by dehydration of α-hydroxyisobutyric acid (HIBA) wherein said method is characterized in that HIBA is reacted in the liquid phase at a temperature of at least 160° C. in the presence of a dehydration catalyst consisting of a metal salt of alpha-hydroxyisobutyric acid. Particularly suitable here are the alkali metal and alkaline earth metal salts of HIBA which are produced in situ in an HIBA melt by reaction of suitable metal salts. This patent describes methacrylic acid yields of up to 95% based on HIBA and the feed for the continuous procedure consists of HIBA and about 1.5 wt % of HIBA alkali metal salt.
Neither of the aforementioned documents describe a method for producing acrylic acid.
WO 2013/155245 filed by Procter & Gamble describes the use of mixtures of phosphates as catalysts in the gas-phase dehydration of lactic acid to afford acrylic acid.
Gas-phase dehydrations require that the reactants are initially converted into the gas phase which often results, for example under the influence of thermal energy on the reactants, in decomposition of the reactants which manifests itself as coking for example.
It is therefore an object of the present invention to provide a method for producing acrylic acid from hydroxycarboxylic acids, which avoids the disadvantages of prior art methods.